Diamond ultraviolet light-emitting device

ABSTRACT

A small, simple current-injection diamond ultraviolet light-emitting device comprising a high-quality diamond grown by chemical vapor deposition (CVD) method ( 1 ), a surface conductive layer ( 2 ) provided on the surface of the diamond, and electrodes ( 4, 5 ) provided on the surface conductive layer. The device is a free-exciton recombination emission diamond ultraviolet light-emitting device comprising a CVD diamond crystal where the free-exciton recombination radiation (235 nm) caused by current injection is dominant.

FIELD OF THE INVENTION

The present invention relates to a diamond ultraviolet light-emittingdevice utilizing a chemical vapor deposition (CVD) diamond that emitsultraviolet, capable of applying to the field of optical informationread/write processing, photolithography, optical processing, phosphorexcitation light source and the like.

SUMMARY OF THE INVENTION

Ultraviolet light characterized in its short wavelength can be utilizedfor micro fabrication, and there are various demands for ultraviolettechnology, such as improving the recording density in the opticalread/write processing or improving the packaging density in the microfabrication of semiconductor devices.

A dutrium lamp and excimer laser are examples of the light source in theultraviolet wavelength region. However, according to the dutrium lamp,the ultraviolet light emission is low in efficiency and brightness. Asfor the excimer laser utilizing gas, the devices are inconvenient to usesince it is large and requires cooing water, and even dangerous since itutilizes hazardous substance (halogen). Therefore, the conventionalultraviolet light sources had various inconveniences for actualapplications.

On the other hand, diamond is also known as a material that emitsultraviolet light. The diamond ultraviolet light-emitting device issmall in size, highly efficient, very bright, and is advantageously safeto use.

The conventional diamond light-emitting device is disclosed for examplein (1) Japanese Patent Laid-Open Publication No. 4-240784, (2) JapanesePatent Laid-Open Publication No. 7-307487, and (3) Japanese PatentLaid-Open Publication No. 8-330624.

These conventional diamond light-emitting devices are formed by dopingboron to the diamond. According to the ultraviolet radiation of theconventional diamond light-emitting device, ultraviolet radiation causedby impurities or lattice defects was dominant, and the free excitonrecombination radiation that emits light having a wavelength as short as235 nm intrinsic to diamond was not dominant. There are explanations onthe free exciton recombination radiation in the above publications, butthey are merely explanations related to the result of radiating electronbeams to the diamond from the exterior and measuring the light emissionby a cathode luminescence (CL) method so as to confirm the properties ofthe diamond.

When considering the diamond ultraviolet light-emitting device, thelight emission caused by impurities/defects has a longer wavelengthcompared to the intrinsic light emission, and therefore it isdisadvantageous for composing a short-wavelength light-emitting device.Furthermore, in order to improve the emission intensity, it is necessaryto introduce to the diamond crystal high density of defects or highconcentration of impurities, and as a result, the quality of the crystalis deteriorated and the intensity of the ultraviolet light emission isreduced. Even further, the introduction of impurities or defects induceda radiation peak at a different wavelength that consumed a portion ofthe injected energy, deteriorating the efficiency of the usefulultraviolet radiation. Because of the above reasons, the light emissionscaused by impurities or defects are not practical for the mechanism of acurrent-injection light-emitting device.

In comparison, the free exciton recombination radiation is a lightemission intrinsic to each material, and generally has the shortestwavelength and a high density of states compared to the variety of otherlight emissions obtained from the material, so it is most preferable increating a practical bright light-emitting device. As for the diamond,the intrinsic spectrum related to the free exciton recombinationradiation is studied using analyzing methods such as the CL method. Theenergy of the free exciton recombination radiation of the diamond atroom temperature corresponds to a wavelength of 229 nm, but actually,light emissions in the phonon side band group that appear near 235 nm,242 nm, 249 nm and 257 nm are mainly observed. In general, all of theabove are included in the term “free exciton recombination radiation”,but the light that is most preferable in an ultraviolet light-emittingdevice is the light having energy around 235 nm, and in the presentspecification, this specific light emission is called the “free excitonrecombination radiation”.

DISCLOSURE OF THE INVENTION

As explained, the conventional diamond ultraviolet light-emitting deviceincludes a light-emitting diamond crystal layer that is poor in quality,containing many impurities or defects. Therefore, even when a diamondcrystal is used to realize a current-injection light-emitting device, itis impossible to obtain sufficient radiation intensity of the freeexciton recombination radiation (wavelength near 235 nm) that is mostadvantageous in practical use.

The object of the present invention is to provide using a chemical vapordeposition (CVD) diamond a current-injection excitation light-emittingdevice in which the free exciton recombination radiation is dominant,the light having the shortest wavelength that is intrinsic to diamond.

For example, according to the spectrum of the light-emitting devicedisclosed in Japanese Patent Laid-Open Publication No. 7-307487, theintensity of the free exciton recombination radiation (235 nm) that isintrinsic to diamond is clearly smaller than the intensity of the boundexcitation radiation (238 nm) that is an example of theimpurity/defect-caused radiation. In comparison, in the case of thediamond light-emitting device of the present invention, the free excitonrecombination radiation is greater than the intensity of the boundexciton radiation caused by impurities/defects, the former being morethan twice greater than the value of the latter in intensity ratio. Thisis the definition of the ultraviolet radiation in which the free excitonrecombination radiation is dominant.

FIG. 10 is referred to in explaining the characteristics of the presentinvention in further detail.

FIG. 10 is a schematic spectra view showing the light emission statuscaused by current injection in the ultraviolet region according to thediamond light-emitting device of the present invention and that of theconventional diamond light-emitting device. The solid line representsthe performance of the diamond light-emitting device according to thepresent invention, and the broken line represents the performance of theconventional diamond light-emitting device.

It is clearly shown in the drawing that the diamond light-emittingdevice according to the present invention shows a main peak at 235 nm,which means that the radiation is caused by the free excitonrecombination radiation, and the main peak is extremely greater than theintensity at 238 nm of the bound exciton radiation caused by boronimpurity.

On the other hand, according to the conventional diamond light-emittingdevice, the main peak is observed at 238 nm the bound exciton radiationcaused by the boron impurity, and shows only small intensity at 235 nmthat corresponds to the free exciton recombination radiation.

In other words, the present invention enables to generate an ultravioletlight emission in which the free exciton recombination radiation isdominant.

In order to achieve the above objects, the present invention proposes inclaim 1 a diamond ultraviolet light-emitting device utilizing a CVDdiamond crystal that is excited and emits light when current is injectedthereto, wherein the free exciton recombination radiation is dominant.

Claim 2 refers to the diamond ultraviolet light-emitting deviceaccording to claim 1 in which the free exciton recombination radiationexited by current injection is dominant, and defines that the peakintensity of the free exciton recombination radiation is greater than atleast twice the peak intensity of other radiations in the wavelengthband of 300 nm or shorter.

Claim 3 refers to the diamond ultraviolet light-emitting deviceaccording to claim 1 or 2, wherein the nitrogen concentration within theCVD diamond crystal is 90 ppm or less.

Claim 4 refers to the diamond ultraviolet light-emitting deviceaccording to any one of claims 1 through 3, wherein the nitrogenconcentration in the plasma when growing the CVD diamond crystal is 200ppm or less in the ratio of nitrogen atoms/carbon atoms.

Claim 5 refers to the diamond ultraviolet light-emitting deviceaccording to any one of claims 1 through 4, wherein the CVD diamondcrystal is monocrystal.

Claim 6 refers to the diamond ultraviolet light-emitting deviceaccording to any one of claims 1 through 5, wherein the CVD diamondcrystal is a CVD diamond crystal grown homoepitaxially.

Claim 7 refers to the diamond ultraviolet light-emitting deviceaccording to any one of claims 1 through 4, wherein the CVD diamondcrystal is polycrystal.

Claim 8 refers to the diamond ultraviolet light-emitting deviceaccording to any one of claims 1 through 7, wherein the CVD diamondcrystal is a crystal at the growth-surface side.

Claim 9 refers to the diamond ultraviolet light-emitting deviceaccording to any one of claims 1 through 8, wherein the CVD diamondcrystal emits the free exciton recombination radiation according tocathodoluminescence spectrum at room temperature.

Claim 10 refers to the diamond ultraviolet light-emitting deviceaccording to any one of claims 1 through 9, wherein the CVD diamondcrystal is characterized in that the intensity ratio of the free excitonrecombination radiation against visible radiation is 0.2 times orgreater according to cathodoluminescence spectrum at −190° C.

Claim 11 refers to the diamond ultraviolet light-emitting deviceaccording to any one of claims 1 through 10, wherein the CVD diamondcrystal comprises a conductive layer at the surface thereof, andelectrodes formed on the conductive layer.

Claim 12 refers to the diamond ultraviolet light-emitting deviceaccording to any one of claims 1 through 11, wherein the CVD diamondcrystal comprises a conductive layer at the surface thereof formed byhydrogen termination, and electrodes formed on the hydrogen terminationlayer.

Claim 13 refers to the diamond ultraviolet light-emitting deviceaccording to any one of claims 1 through 12, wherein conductivity isprovided to the CVD diamond crystal by doping boron thereto.

Claim 14 refers to the diamond ultraviolet light-emitting deviceaccording to any one of claims 1 through 13, wherein the boronconcentration within the CVD diamond is 60 ppm or less.

Claim 15 refers to the diamond ultraviolet light-emitting deviceaccording to any one of claims 1 through 14, wherein the concentrationof boron within the plasma when growing the CVD diamond crystal is 1000ppm or less in ratio of boron atoms/carbon atoms.

Claim 16 refers to the diamond ultraviolet light-emitting deviceaccording to any one of claims 1 through 15, wherein the effectiveacceptor concentration within the CVD diamond crystal is 20 ppm or lessin quantification based on infrared absorption spectroscopy.

Claim 17 refers to the diamond ultraviolet light-emitting deviceaccording to any one of claims 1 through 16, wherein according to theCVD diamond crystal, the free exciton recombination radiation is 0.1times or greater in peak intensity than the boron bound excitonrecombination radiation according to cathodoluminescence spectrum at−190° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram showing the diamond ultravioletlight-emitting device utilizing the hydrogen terminated diamondpolycrystal film according to the first embodiment of the presentinvention;

FIG. 2 is an explanatory view showing the manufacturing steps of thediamond ultraviolet light-emitting device utilizing the CVD diamondcrystal according to the present invention;

FIG. 3 is a characteristics chart showing the free exciton recombinationradiation (ultraviolet radiation) by injecting current to the diamondultraviolet light-emitting device according to the present invention;

FIG. 4 is a conceptual diagram showing the structure of the diamondultraviolet light-emitting device utilizing the hydrogen terminateddiamond monocrystal film according to the second embodiment of thepresent invention;

FIG. 5 is a CL spectrum view showing the diamond monocrystal filmaccording to the present invention;

FIG. 6 is a characteristics chart showing the free exciton recombinationradiation (ultraviolet radiation) by injecting current to the diamondultraviolet light-emitting device utilizing the hydrogen terminateddiamond monocrystal film according to the present invention;

FIG. 7 is a current-voltage characteristics chart of the diamondultraviolet light-emitting device utilizing the hydrogen-terminateddiamond monocrystal layer according to the present invention;

FIG. 8 is a conceptual diagram showing the structure of the diamondultraviolet light-emitting device using the boron-doped diamondmonocrystal film according to the third embodiment of the presentinvention;

FIG. 9 is a characteristics chart showing the free exciton recombinationradiation (ultraviolet radiation) by injecting current to the diamondultraviolet light-emitting device utilizing the boron-doped diamondmonocrystal film according to the present invention; and

FIG. 10 is a light emission characteristics chart explaining theintensity of the free exciton recombination radiation of the diamondultraviolet light-emitting device utilizing the CVD diamond according tothe present invention and that of the prior-art diamond ultravioletlight-emitting device.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The structure of a diamond ultraviolet light-emitting device utilizing aCVD diamond according to the preferred embodiment of the presentinvention will now be explained in detail with reference to FIG. 1.

FIG. 1 (A) is a planery view showing a typical state where multipleelectrodes are formed on a diamond crystal substrate, and FIG. 1 (B) isa cross-sectional view showing an enlarged view of the cross-sectiontaken at line B—B of FIG. 1 (A). This embodiment is hereinafter calledembodiment 1.

The present ultraviolet light-emitting device utilizing the free-excitonrecombination radiation is formed using a diamond polycrystal filmhaving hydrogen-termination performed to the surface thereof.

A diamond ultraviolet light-emitting device 10 utilizing thishydrogen-terminated diamond polycrystal film comprises a high-qualitydiamond crystal layer 1 obtained by the CVD method and including only aslight amount of lattice defects or impurities other than the dopant, asurface conductive layer 2 that is formed by processing the surface ofthe diamond crystal layer 1 with a hydrogen-termination process so as toprovide electrical conductivity thereto, and first electrodes 4 andsecond electrodes 5 formed on the hydrogen-terminated layer. The firstelectrodes 4 and second electrodes 5 are each formed of a chromium (Cr)layer 31 formed on the surface of the hydrogen-termination layer 2 so asto provide good adhesiveness with the diamond, and a gold (Au) layer 32formed on top of the Cr layer.

The diamond crystal layer 1 forming the diamond ultravioletlight-emitting device 10 using the hydrogen-terminated diamondpolycrystal film is formed on a molybdenum (Mo) substrate by aninductively-coupled radiofrequency thermal plasma CVD method (RF thermalplasma CVD method). The present diamond crystal layer 1 is formed as anundoped CVD crystal, fabricating a high-quality diamond crystal thatonly includes a slight amount of impurity or lattice defect.

The present diamond thick-film is manufactured under the followingconditions of growth.

[Conditions of Growth]

-   -   RF input: 45 kW    -   Pressure: 200 Torr    -   Gas: Ar; 31.8 liter/min, H₂; 11.2 liter/min, CH₄; 0.17 liter/min    -   Gas purity: Ar; 99.9999%, H₂; 99.99999%, CH₄; 99.9999%    -   Growth time: 30 hours    -   Substrate temperature: approx. 900° C.    -   Substrate: molybdenum (Mo) plate, diameter: 50 mm, thickness: 5        mm    -   Film thickness: 100 μm

After growing the film, the Mo substrate on which the diamond thick-filmis grown is dissolved using acid, thereby obtaining an independentdiamond film.

Raman scattering spetroscopy will now be explained. Raman scattering isa phenomenon where the photon radiated into the sample interacts withthe phonon within the crystal and is re-emitted as a photon having alower energy than the initial by the energy of the phonon. A Ramanscattering spectroscopy is a sample evaluation method by analyzingspectrum of the radiated light (scattered light), so as to obtaininformation such as the lattice defect, stress, impurities and the likein the crystal structure.

In a diamond crystal, the intrinsic Raman scattering peak appearsbetween 1332 [1/cm] and 1333 [1/cm]. The full width at half maximum ofthe peak is especially sensitive to lattice defect density and nitrogendensity, and becomes narrower as the lattice defect density or theimpurity concentration other than the dopant such as nitrogen becomessmaller. Therefore, Raman scattering is effective for evaluating thecrystal of the ultraviolet light-emitting device wherein the freeexciton recombination radiation excited by injecting current in thediamond is emitted as the main light emission.

As a result of the measurement using the Raman spectroscopy wherein anAr laser excitation (wavelength approximately 514.5 nm) is used tomeasure the Raman spectrum, the full width at half maximum of thediamond intrinsic scattering peak near the Raman shift 1332-1333 [1/cm]became narrower near the upper portion of the film cross-section thanthe lower portion. At the uppermost area of the film, the full width athalf maximum of the intrinsic peak became smaller than approximately 2.0[1/cm]. That is, the crystallinity of the diamond is more advantageousat the growth surface side than the back surface of the film near thesubstrate. In this example, the full width at half maximum of the peakvalue is called FWHM.

The following is an explanation on the CL measurement. CL measurement isa sample evaluation method where electron beam is radiated on a sampleplaced in vacuum, and the light emitted from the sample is measured asspectrum. The spectrum varies according to the sample temperature.

The free exciton recombination radiation measured by the aboveevaluation method is blocked by the existence of impurities or lowconcentration of lattice defect, so when the intensity of the lightemission increases, the completeness of the diamond is considered to behigher.

According to the evaluation obtained by measuring the emission spectrumaccording to the CL method, the ultraviolet light emission by the freeexciton recombination radiation which is intrinsic to diamond isstronger at the growth surface side (upper side) of the diamond, and thevisible light emission caused by impurities or defects is stronger nearthe side adjoining the substrate (lower side). In other words, thegrowth surface of the polycrystal diamond has higher quality with littledefects, but crystallinity of the surface adjacent to the substrate ismuch lower.

The diamond layer (polycrystal diamond) 1 manufactured by theabove-mentioned RF thermal plasma CVD method has a flat surface thatcontacts the substrate, but the growing surface is rough and uneven, sothere is a need to polish the growing surface using a diamond paste soas to smoothen the surface.

The diamond film 1 having both surfaces smoothed is cut into aplate-like shape with a size of approximately 1 mm+2 mm.

This undoped CVD diamond film 1 is insulative, so it is necessary toprovide a hydrogen termination process to the surface thereof withinhydrogen plasma using a microwave plasma chemical vapor depositionequipment (MW-CVD) in order to provide electric conductivity to thesurface thereof. By this process, the surface of the diamond film 1becomes hydrogen-terminated and comprises conductivity.

The method of hydrogen-terminating the surface of the CVD diamond filmis disclosed for example in Japanese Patent Laid-open publication No.8-139109. Here, hydrogen-termination refers to a state where hydrogenatoms are bonded to the dangling bonds of carbon atoms on the surface ofthe diamond crystal being grown. For example, a hydrogen-terminateddiamond crystal can be obtained by processing the diamond crystal inhydrogen plasma.

The surface hydrogen-terminating process is performed under theconditions mentioned below.

[Conditions for Surface Hydrogen Terminating Process]

-   -   MW output: 600 W    -   Hydrogen flow rate: 500 ml/min    -   Processing time: 10 min    -   Substrate temperature: 900° C.    -   Cooling time: 30 min

Hydrogen gas is continuously flown even during cooling time, so as toimprove the completeness of the process.

Electrodes are formed on the hydrogen-terminated layer of the diamondcrystal film. The manufacturing steps of the electrodes will now beexplained with reference to FIG. 2.

After growing a diamond film on a molybdenum (Mo) substrate using CVDmethod, the polished surface 11 of the CVD diamond film 1 removed fromthe substrate and having high quality is hydrogen-terminated using theabove process to form a hydrogen terminated layer 2 (FIG. 2 (A)).

Sputtering is performed to the diamond film 1 in a direct currentsputtering equipment with chromium target (Cr) for 20 seconds at asubstrate temperature of 200° C., a bias voltage of 500 V and current of1 A, thereby creating a Cr layer 31 having a thickness of 500 Å on thehydrogen-terminated layer 2. Thereafter, sputtering is performed theretotargeting gold (Au) for five minutes with a substrate temperaturemaintained at 200° C., 700 V and 1 A, thereby creating an Au layer 32having a thickness of 2000 Å on the Cr layer 31 (FIG. 2 (B)).

Next, a positive photoresist layer is formed on the Au layer 32 forexample by spin coating, which is dried for 30 minutes in the atmosphereat 80° C., before exposing ultraviolet light to the unnecessary areasusing a mask aligner. Then, the exposed areas are removed using aphotoresist developing solution, and then dried for 30 minutes in theatmosphere at 130° C., thereby creating the resist mask 6 (FIG. 2 (C)).This process is performed for example under the following conditionswhere the photoresist viscosity is 100 Cp, the revolution is 3500 rpm,for 15 seconds, with an ultraviolet exposure of 200 mJ/cm².

This resist mask 6 is used to etch the Au layer 32 and the Cr layer 31,in order to fabricate first electrodes 4 and second electrodes 5 (FIG. 2(D)). The etching of the Au layer 32 is performed using anmonium iodidesolution, and the etching of the Cr layer 31 is performed using ceriumanmonium secondary nitrate solution.

Thereafter, acetone is used to remove the resist 6, and then a diamondultraviolet light-emitting device 10 is formed using thehydrogen-terminated diamond polycrystal film as shown in FIG. 1.

The result of the measurement of current-injection light emission of thediamond ultraviolet light-emitting device 10 utilizing thehydrogen-terminated diamond polycrystal film is explained with referenceto FIG. 3.

FIG. 3 shows the measured result of the free exciton recombinationradiation spectrum that explains the state of the free excitonrecombination radiation, wherein the horizontal axis represents thewavelength (nm) and the vertical axis represents the light emissionintensity (arbitrary scale).

The sample shows higher crystallinity at the growth surface side whengrowing the diamond film, so the electrodes are formed to the growthsurface side. The measurement is performed by providing a direct currentvoltage of 220 V between the adjacent electrodes 4 and 5, and flowing acurrent of 1.0 mA thereto.

As is clearly shown in FIG. 3, the peak exists around 235 nm, and thiswavelength can be identified as the ultraviolet light emission of thefree exciton recombination radiation caused by current injection. In thewavelength range below 300 nm, the light emission peak caused byimpurities or lattice defects is below the detectable intensity limit.From the result of FIG. 3, according to a diamond polycrystal filmhaving a hydrogen-terminated surface, the sample having a highcrystallinity shows dominant ultraviolet light emission of the freeexciton recombination radiation caused by current injection, that couldnot be obtained according to the prior art device. According to theprior art, the ultraviolet light emission was mainly caused byimpurities or defects. However, the present invention characterizes inthat the free exciton recombination radiation is dominant.

The following explains the diamond ultraviolet light-emitting deviceutilizing a hydrogen-terminated diamond monocrystal film, which is a CVDdiamond crystal epitaxially grown on a diamond substrate by microwaveplasma chemical vapor deposition method (MW-CVD method). Hereinafter,this embodiment is called embodiment 2.

The structure of the diamond ultraviolet light-emitting device 100utilizing the hydrogen-terminated diamond monocrystal film according toembodiment 2 will now be explained with reference to FIG. 4.

FIG. 4 is a cross-sectional view showing a simplified and enlargedpartial cross section of the diamond ultraviolet light-emitting device100 utilizing the hydrogen-terminated diamond monocrystal, the planeryview of which is similar to FIG. 1 (A) and is omitted.

The diamond ultraviolet light-emitting device 100 utilizing thehydrogen-terminated diamond monocrystal is manufactured byhomoepitaxially growing a high quality diamond monocrystal film 13 onthe surface of a high-pressure synthesized diamond crystal 12, thenproviding a hydrogen-terminated layer 2 having electrical conductivityon the surface of the diamond monocrystal film 13 by providing ahydrogen-termination process thereto, and finally forming electrodes 4and 5 on the hydrogen-terminated layer 2 in a similar manner explainedin embodiment 1 of FIG. 1. Each electrode 4, 5 comprises a chromiumlayer 31 and a gold layer 32.

The formation of the homoepitaxially grown diamond crystal film isrealized under the following conditions utilizing a MW-CVD equipment.

[Conditions of Growth]

-   -   Substrate: 1b (100) high-pressure diamond crystal    -   MW output: 500 W    -   Gas: CH₄; 0.2% in H₂, flow rate 500 ml/min    -   Gas purity: CH₄; 99.9999%, H₂; 99.99999%    -   Temperature: 870° C.    -   Growth pressure: 40 Torr    -   Growth time: 64.5 hours

After growth, hydrogen is used for processing the crystal for 10minutes, before cooling it in hydrogen gas.

Since the substrate is made of diamond, the diamond crystal obtained bythe above-mentioned process cannot be removed from the substrate, andthe homoepitaxial film formed by the above process is used together withthe substrate in the following processes and measurement.

Since the diamond crystal obtained by the above process is undoped andinsulative, hydrogen-termination is performed to the surface thereofunder the following conditions so as to provide electrical conductivitythereto.

[Conditions for Surface Hydrogen-Termination Process]

-   -   Pressure: 40 Torr    -   MW output: 600 W    -   Substrate temperature: 900° C.    -   Processing time: 10 min    -   Cooling time: 30 min

Thereafter, electrodes 4 and 5 are formed on the surface of thehydrogen-terminated layer 2 of the hydrogen terminated diamondmonocrystal 13 obtained by the above process. The manufacturing stepsare similar to those disclosed in FIG. 2.

The method for evaluating the quality of the diamond monocrystalobtained by the CL method is explained with reference to FIG. 5.

FIG. 5 is a chart showing the wavelength in the horizontal axis (nm:ultraviolet range—visible radiation range), and the vertical axis showsthe radiation intensity of the CL in arbitrary scale measured at atemperature of −190° C. in vacuum.

As could be seen from FIG. 5, in the crystal evaluation by CL of thediamond monocrystal film grown homoepitaxially, the free excitonrecombination radiation is recognized in the ultraviolet range, butalmost no radiation is observed in the visible radiation range caused bylattice defects, so it is clear that the present film has highcrystallinity.

The following explains the current-injection radiation characteristicsof the diamond ultraviolet light-emitting diode 100 utilizing thepresent hydrogen-terminated diamond monocrystal film.

The current injection radiation is measured by providing a directcurrent voltage of 200 V between the adjacent electrodes 4 and 5, andthen providing 50 μA.

FIG. 6 explains the state of the free exciton recombination radiationutilizing the hydrogen-terminated diamond monocrystal film, and it showsthe spectrum of the free exciton recombination radiation in theultraviolet range caused by current injection (EL) under roomtemperature, wherein the horizontal axis shows the wavelength (nm) andthe vertical axis shows the radiation intensity (arbitrary scale). Ascould be seen from the drawing, in a diamond ultraviolet light-emittingdevice utilizing the hydrogen-terminated MW-CVD homoepitaxially growndiamond monocrystal film, a clear peak exists near 235 nm, and the freeexciton recombination radiation is observed. In the range belowwavelength of 300 nm, the radiation peaks caused by impurities orlattice defects are below a detectable intensity limit.

FIG. 7 shows the voltage-current characteristics of the diamondultraviolet light-emitting device 100 utilizing the hydrogen-terminatedMW-CVD homoepitaxially grown diamond monocrystal film shown in FIG. 4.

As a result, even in the diamond ultraviolet light-emitting device 100utilizing the hydrogen-terminated diamond monocrystal film, anultraviolet light-emitting diode is obtained where the free excitonrecombination radiation caused by current injection under roomtemperature is dominant.

Next, an example is explained where MW-CVD method is used to dope boron(B) and to realize homoepitaxial growth on a high-pressure synthesizeddiamond crystal substrate, thereby creating a boron-doped MW-CVDhomoepitaxial diamond crystal to be applied to the diamond ultravioletlight-emitting device.

When boron is doped, the diamond becomes a p-type semiconductor, and isprovided with electric conductivity. However, in the present invention,the doping of boron is performed so as to realize a current-injectiondevice by providing electric conductivity to the crystal, and not toobtain a radiation caused by boron. This embodiment is hereinaftercalled embodiment 3.

The structure of the diamond ultraviolet light-emitting device 110utilizing the boron-doped diamond monocrystal film according toembodiment 3 will be explained with reference to FIG. 8.

FIG. 8 is a cross-sectional view showing the simplified partiallyenlarged cross section of the diamond ultraviolet light-emitting device110 utilizing the boron-doped diamond monocrystal, and the planery viewthereof is similar to FIG. 1 (A) so it is omitted.

The diamond ultraviolet light-emitting device 110 utilizing theboron-doped diamond monocrystal is manufactured by homoepitaxiallygrowing a high-quality boron-doped diamond monocrystal film 21 on thesurface of the high-pressure synthesized diamond crystal 12, and thenforming electrodes 4 and 5 on top of the boron-added diamond monocrystalfilm 21 in a manner similar to embodiment 1 shown in FIG. 1. Each of theelectrodes 4 and 5 comprises a chromium layer 31 and a gold layer 32.

The creation of the boron-doped diamond monocrystal film byhomoepitaxial growth is performed using the MW-CVD equipment and underthe following conditions.

[Conditions of Growth]

-   -   Substrate: 1b (100) high-pressure synthesized diamond crystal    -   MW output: 600 W    -   Gas: CH₄; 0.1 % in H₂, Boron (B); 50 ppm (=B/C), flow rate 500        ml/min    -   Gas purity: CH₄; 99.9999,%, H₂; 99.99999%    -   Temperature: 900° C.    -   Growth pressure: 40 Torr    -   Growth time: 135 hours    -   Process:

Before growth; after cleaning substrate with ethanol, performing H₂plasma treatment for 10 minutes After growth; performing H₂ plasmatreatment for 10 min, then cooling in H₂.

The boron-doped diamond monocrystal film formed according to the aboveconditions is boiled in sulfuric acid and hydrogen peroxide mixture(H₂SO₄+H₂O₂) to remove the surface conductive layer, and then a diamondultraviolet light-emitting device utilizing the boron-doped MW-CVDhomoepitaxially grown diamond crystal film is formed similar to theprocess shown in FIG. 2.

In the diamond ultraviolet light-emitting device 110 utilizing theboron-doped diamond monocrystal film, a DC voltage of 150 V is appliedbetween electrodes 4 and 5, so as to perform current injection ofapproximately 1 mA.

The current injection spectrum under room temperature of the diamondultraviolet light-emitting device utilizling the above-mentionedboron-doped diamond monocrystal film is explained with reference to FIG.9. As shown in FIG. 9, an ultraviolet radiation having a peak at 235 nmcaused by the free exciton recombination radiation is also obtained bythe present device. In the wavelength range below 300 nm, the radiationpeak caused by impurities or lattice defects are below a detectableintensity limit.

According to the prior art ultraviolet light-emitting device utilizingthe boron-doped diamond crystal, an ultraviolet light-emission caused bythe boron-derived bound exciton radiation was dominant. On the otherhand, the above-mentioned experiment proved that the diamond ultravioletlight-emitting device utilizing the boron-doped diamond monocrystal filmaccording to the present embodiment characterized in that theultraviolet radiation caused by the free exciton recombination radiationcaused by current injection is dominant. In other words, according tothe present embodiment, boron is doped to the diamond crystal film so asto provide electric conductivity thereto, and since the diamond crystalhas a high crystallinity, ultraviolet radiation is obtained by currentinjection as the free exciton recombination radiation intrinsic todiamond.

Embodiment 3 utilized boron-doped diamond monocrystal film formed bygrowing boron-doped diamond crystal film on a diamond substurate, butother substrates such as silicon or metal can be used instead, and theboron-doped diamond crystal film can be either monocrystal orpolycrystal.

The following explains the result of the crystal evaluation by the CLmethod of the diamond ultraviolet light-emitting device utilizing theCVD diamond crystal according to the present invention, and whether thefree exciton recombination radiation caused by current injection isobserved or not, with reference to Table 1.

Table 1 shows the radiation characteristics of the diamond ultravioletlight-emitting device utilizing the CVD diamond crystals manufacturedthrough RF thermal plasma CVD method and MW-CVD method.

Here, the light-emitting devices of the sample of embodiments 4 and 5and the comparison example 1 are manufactured by MW-CVD method similarto embodiment 2. Only the following conditions of composition differ ineach manufacturing process.

-   -   Embodiment 4: Methane concentration; 1.0%, MW output; 500 W    -   Embodiment 5: Methane concentration; 0.30%, MW output; 500 W    -   Comparison example 1: Methane concentration; 0.50%, MW output;        400 W

Further, comparison examples 2 and 3 are manufactured by RF-CVD methodsimilar to embodiment 1. The only point that differs from embodiment 1is that both comparison examples utilize the surface of the diamondfree-standing film positioned adjacent to the substrate. The comparisonexample 2 is used as it is without polishing, and comparison example 3is used after polishing the surface for about 10 μm.

TABLE 1 Crystallinity evaluation by CL spectrum measurement Ratio ofintensity between free exciton Free exciton recombination Free excitonrecombination radiation/visible radiation at radiation by radiation atroom current Sample −190° C. temperature injection Embodiment 1 0.2 YesYes Embodiment 2 6.7 Yes Yes Embodiment 4 34 Yes Yes Embodiment 5 >100Yes Yes Comparison 0.16 No No example 1 Comparison 0.01 No No example 2Comparison 0.03 No No example 3

In Table 1, the second and third columns show crystallinity evaluationsbased on CL spectrum measurement, wherein the second column shows thecomparison (FE/VB) between the peak intensity FE of the free excitonrecombination radiation and the peak intensity VB of visible radiationcaused by impurities and defects at −190° C., and the third column showswhether the free exciton recombination radiation was observed or not atroom temperature. The fourth column shows whether the free excitonrecombination radiation caused by current injection was observed or not.

As can be seen from Table 1, in comparison examples 1 through 3,according to the crystallinity evaluation based on the CL methodperformed at a temperature of −190° C., the ratio FE/VB is 0.16, 0.01,and 0.03, respectively, but according to the crystallinity evaluationperformed at room temperature, no free exciton recombination radiationcould be observed in the comparison examples. In this case, no freeexciton recombination radiation by current injection was observed atroom temperature.

On the other hand, as for the diamond ultraviolet light-emitting device(embodiments 1, 2, 4, 5) of the present invention, according to thecrystallinity evaluation by the CL method performed at −190° C., theratio FE/VB is 0.2, 6.7, 34, and over 100, respectively. Further,according to the crystallinity evaluation by the CL method performed atroom temperature, the free exciton recombination radiation is clearlydetected in all the examples. Accordingly, the free excitonrecombination radiation is clearly obtained by current injection at roomtemperature.

Based on the above results, it is understood that in order to obtain thediamond ultraviolet light-emitting device, the free excitonrecombination radiation intensity must be over 0.2 times the visiblelight-emitting intensity in the CL spectrum at a temperature of −190° C.It is further understood that in the CL spectrum at room temperature,the free exciton recombination radiation must be observed.

The following is an explanation of the nitrogen concentration dependenceof the free exciton recombination radiation in the CVD diamond crystalultraviolet light-emitting device caused by current injection.

Table 2 shows the measured result of the relation between the amount(ratio) of nitrogen atoms being introduced to the amount of carbon atomsin the atmosphere when growing the CVD diamond crystal for the diamondultraviolet light-emitting device, and whether the current-injectionfree exciton recombination radiation is observed or not.

In Table 2, embodiment 2 and comparison examples 4 and 5 all utilize ahydrogen terminated MW-CVD homoepitaxial diamond monocrystal film. Theonly difference in the manufacturing method for each example is theamount of nitrogen added to the plasma atmosphere based on the followingconditions.

-   -   Embodiment 2: Ratio of nitrogen atoms/carbon atoms; below 100        ppm    -   Embodiment 6: Ratio of nitrogen atoms/carbon atoms; 200 ppm    -   Comparison Example 4: Ratio of nitrogen atoms/carbon atoms; 2000        ppm    -   Comparison Example 5: Ratio of nitrogen atoms/carbon atoms;        20000 ppm

Further, there is no intended addition of nitrogen in embodiment 2, andthe described nitrogen concentration is calculated based on the resultof analysis of the components of the methane gas and the hydrogen usedfor the growth, and the leak quantity of the CVD equipment.

TABLE 2 Nitrogen concentration in plasma during growth Free exciton(ratio of nitrogen recombination atoms/carbon atoms, radiation caused bySample ppm) current injection Embodiment 2 under 100 Yes Embodiment 6200 Yes Comparison example 4 2000 No Comparison example 5 20000 No

Based on the measured results, it is understood that the number ofnitrogen atoms in the atmosphere during growth of the CVD diamondcrystal must be equal to or less than 200 ppm in ratio to carbon atomsin order to generate current injection free exciton recombinationradiation.

Moreover, the nitrogen concentration within the sample of embodiment 6is measured by secondary ion mass spectroscopy, the result of whichturned out to be 90 ppm. Accordingly, it is understood that in order tofabricate a diamond ultraviolet light-emitting device, the nitrogenconcentration within the crystal must be equal to or less than 90 ppm.

The following is an explanation on the boron concentration dependency ofthe free exciton recombination radiation of the CVD diamond crystalultraviolet light-emitting device based on current injection.

Tables 3 through 5 show for example the measured result of the relationbetween the number (ratio) of boron atoms being introduced to the numberof carbon atoms in the atmosphere when growing the CVD diamond crystalfor the diamond ultraviolet light-emitting device, and whether thecurrent-injection free exciton recombination radiation is observed ornot.

Table 3 shows the ratio B/C of the number of boron atoms and the numberof carbon atoms within the plasma during the step, and whether thecurrent-injection free exciton recombination radiation is observed ornot.

Table 4 shows the active acceptor concentration (ppm) within the crystalbased on an infrared absorption (IR) spectroscopy, and whether thecurrent-injection free exciton recombination radiation is observed ornot.

Table 5 shows the intensity ratio (FE/BE) of the free excitonrecombination radiation and the boron-derived bound excitonrecombination radiation based on CL spectrum at −190° C., and whetherthe current-injection free exciton recombination radiation is observedor not.

TABLE 3 Boron concentration in plasma during growth Free exciton (ratioof boron recombination atoms/carbon atoms, radiation caused by Sampleppm) current injection Embodiment 3 50 Yes Embodiment 7 1000 YesComparison example 6 1500 No Comparison example 7 3000 No Comparisonexample 8 5000 No

TABLE 4 Active acceptor Free exciton concentration recombination withincrystal by IR radiation caused by Sample measurement (ppm) currentinjection Embodiment 3 2 Yes Embodiment 7 20 Yes Comparison example 6above measurement No limit Comparison example 7 above measurement Nolimit Comparison example 8 above measurement No limit

TABLE 5 Free exciton recombination FE/BE ratio (*) in CL radiationcaused by Sample spectrum at −190° C. current injection Embodiment 3 2.3Yes Embodiment 7 0.10 Yes Comparison example 6 0.03 No Comparisonexample 7 Below measurement No limit Comparison example 8 Belowmeasurement No limit * ratio of intensity of free exciton recombinationradiation/boron-derived bound exciton recombination radiation

All the samples used for the above measurement are grown by MW-CVDmethod, and each homoepitaxially grown diamond film is manufacturedunder the same conditions listed below, except for the boronconcentration.

-   -   Substrate: 1b (100) high-pressure synthesized diamond crystal    -   MW output: 600 W    -   Gas: CH₄; 0.1-1.0% in H₂, flowrate; 500 ml/min    -   Temperature: 850-900° C.    -   Pressure: 40 Torr

In the above Table, B/C shows the amount of trimethyl boron (B(CH₃)₃)introduced to material methane (CH₄) during CVD process, which isrepresented by the number ratio of boron atoms/carbon atoms.

In order to quantify the boron concentration, a quantification methodusing infrared absorption (IR) spectroscopy under room temperature (P.M. Cherenko, H. M. Strong and R. E. Tuft, Phil. Mag., vol. 23, p313,1971) is utilized to measure the effective acceptor concentration.

Actually, the infrared absorption spectrum of the sample is measuredusing a micro IR equipment, and the absorbance a₁ at wave number of 1280[1/cm] or the absorbance a₂ at a wave number of 2800 [1/cm] iscalculated. When the boron concentration within the crystal isapproximately 10 ppm or more, the former (a₁) is effective, and when itis less than 10 ppm, the latter (a₂) is effective.

The conversion formula of the former and latter values is represented bya₂/a₁=22.

When the film thickness is represented by d (cm), the effective acceptorconcentration (N_(A)) in the crystal can be calculated by the followingequation.N _(A) =0.086×a ₁ /d

Secondary ion mass spectroscopy (SIMS) is used to quantify the boronconcentration within the diamond crystal accurately. By utilizing theSIMS method, the total boron concentration including the boron atomsthat are inactive as acceptor can be measured. The boron within thediamond crystal forms an acceptor level at approximately 350 meV abovethe valence band, and some of them become activated at room temperatureand function as acceptors. Therefore, the effective acceptorconcentration according to the infrared spectroscopy at room temperatureis smaller compared to the total boron concentration.

The intensity ratio of intensity of the free exciton recombinationradiation and the boron-derived bound exciton recombination radiation(FE/BE ratio) measured by CL method at −190° C. indirectly representsthe boron concentration within the diamond crystal. (Refer to H.Kawarada et al., Physical Review B, vol. 47, p. 3633-3637 (1993).)

The samples are grown by varying the ratio of boron atoms against thecarbon atoms within the atmosphere when growing the CVD diamonds, andtests are performed for example to check whether the current-injectionfree exciton radiation is observed or not.

In embodiment 3 the B/C ratio during growth is 50 ppm, in embodiment 7the B/C ratio during growth is 1000 ppm, in comparison example 6 the B/Cratio during growth is 1500 ppm, in comparison example 7 the B/C ratioduring growth is 3000 ppm, and in comparison example the B/C ratioduring growth is 5000 ppm.

In embodiments 3 and 7, the free exciton recombination radiation basedon current injection is observed, but in the comparison examples 6, 7and 8, no free exciton recombination radiation based on currentinjection is observed.

Based on the above fact, it is understood that in order to obtain thediamond ultraviolet light-emitting device, the ratio of boron atoms tocarbon atoms within the plasma during growth must be 1000 ppm or less.

As a result of quantification of the active acceptor concentrationwithin the crystal utilizing the IR method, the concentration ofembodiment 3 is 2 ppm and the concentration of embodiment 7 is 20 ppm.However, the boron concentration according to comparison examples 6, 7and 8 are above the limit of measurement by the IR method.

Accordingly, it is understood that in order to obtain the diamondultraviolet light-emitting device, the active acceptor concentrationwithin the crystal according to the IR measurement method must be 20 ppmor less.

The peak intensity ratio (FE/BE) of the free exciton recombinationradiation and the boron-derived bound exciton recombination radiation ismeasured based on CL method at −190° C., and the result is 2.3 forembodiment 3, 0.10 for embodiment 7, 0.03 for comparison example 3, andbelow the limit of measurement for both comparison examples 7 and 8.

Accordingly, it is understood that in order to obtain the diamondultraviolet light-emitting device, the peak intensity ratio (FE/BE) ofthe free exciton recombination radiation to the boron-derived boundexciton recombination radiation measured by CL at −190° C. must be 0.10or higher, or in other words, the ratio must be 0.1 times or higher.

The boron concentration of the samples of embodiment 7 and comparisonexample 6 are analyzed by SIMS, wherein the former is 60 ppm and thelatter is 300 ppm.

Therefore, in order to obtain the diamond ultraviolet light-emittingdevice, the boron concentration within the crystal must be 60 ppm orless according to the analysis by the SIMS method.

The prior art lacks to propose manufacture method of a current injectiondiamond light-emitting device by intentionally limiting the nitrogenconcentration or the boron concentration within the crystal. This isbecause the conventional diamond light-emitting device only emittedultraviolet light caused by impurities or lattice defects, and did notutilize the free exciton recombination radiation that is intrinsic todiamond and is utilized in the present invention.

As explained, the present invention provides a diamond ultravioletlight-emitting device utilizing a CVD diamond in which thecurrent-injection free exciton recombination radiation is dominant.

In the detailed description, the insulative diamond crystal film 1 caneither be monocrystal or polycrystal.

Further, electric conductivity can be provided to the diamond either byhydrogen-termination of the crystal surface or by doping boron so as tocreate a p-type semiconductor.

INDUSTRIAL APPLICABILITY

As explained, the diamond ultraviolet light-emitting device utilizingthe CVD diamond crystal according to the present invention can generatea short-wavelength ultraviolet radiation caused by current injection.

1. A diamond ultraviolet light-emitting device, comprising: a diamondcrystal formed by a chemical vapor deposition (CVD) process; a firstelectrode formed on the diamond crystal; and a second electrode formedon the diamond crystal, wherein when current injection is performedbetween the first and second electrode, the diamond ultravioletlight-emitting device dominantly emits a light of a free excitonrecombination radiation, having a peak wavelength of 235 nm, 242 nm, 249nm, or 257 nm.
 2. A diamond ultraviolet light-emitting device accordingto claim 1 in which the free exciton recombination radiation excited bycurrent injection is dominant, wherein the peak intensity of the freeexciton recombination radiation is stronger than at least twice the peakintensity of other radiations in the wavelength region below 300 nm. 3.A diamond ultraviolet light-emitting device according to claims 1,wherein the nitrogen concentration within said CVD diamond crystal is 90ppm or less.
 4. A diamond ultraviolet light-emitting device according toclaim 1, wherein the diamond crystal is formed under a plasma having anitrogen concentration of 200 ppm or less in a ratio of nitrogenatoms/carbon atoms.
 5. A diamond ultraviolet light-emitting deviceaccording to claim 1, wherein said CVD diamond crystal is monocrystal.6. A diamond ultraviolet light-emitting device according to claim 1,wherein said CVD diamond crystal is a CVD diamond crystal grownhomoepitaxially.
 7. A diamond ultraviolet light-emitting deviceaccording to claim 1, wherein said CVD diamond crystal is polycrystal.8. A diamond ultraviolet light-emitting device according to claim 1,wherein the first electrode and/or the second electrode is formed on asurface of the diamond crystal, the surface being a growing surface inthe chemical vapor deposition (CVD) process.
 9. A diamond ultravioletlight-emitting device according to claim 1, wherein when the diamondultraviolet light-emitting device is subjected to a cathode luminescencespectrum method at room temperature, the diamond ultravioletlight-emitting device emits the free exciton recombination radiation.10. A diamond ultraviolet light-emitting device according to claim 1,wherein when the diamond ultraviolet light-emitting device is subjectedto a cathode luminescence spectrum method at −190° C., the diamondultraviolet light-emitting device emits the free exciton recombinationradiation at an intensity ratio (FE/VE) of 0.2 or more, wherein FErepresent a peak intensity of the free exciton recombination radiation,and VE represents a peak intensity of a visible radiation.
 11. A diamondultraviolet light-emitting device according to claim 1, wherein said CVDdiamond crystal comprises a conductive layer at the surface thereof, andelectrodes formed on said conductive layer.
 12. A diamond ultravioletlight-emitting device according to claim 1, wherein said CVD diamondcrystal comprises a conductive layer at the surface thereof formed byhydrogen-termination, and electrodes formed on said hydrogen terminationlayer.
 13. A diamond ultraviolet light-emitting device according toclaim 1, wherein conductivity is provided to said CVD diamond crystal bydoping boron thereto.
 14. A diamond ultraviolet light-emitting deviceaccording to claim 1, wherein the boron concentration within said CVDdiamond is 60 ppm or less.
 15. A diamond ultraviolet light-emittingdevice according to claim 1, wherein chemical vapor deposition (CVD)diamond crystal is formed under a plasma having a boron concentration of1000 ppm or less in a ratio of boron atoms/carbon atoms.
 16. A diamondultraviolet light-emitting device according to claim 1, wherein thediamond crystal has an effective acceptor concentration of 20 ppm orless based on infrared absorption spectroscopy.
 17. A diamondultraviolet light-emitting device according to claim 1, wherein when thediamond ultraviolet light-emitting device is subjected to a cathodeluminescence spectrum method at −190° C., the diamond ultravioletlight-emitting device emits the free exciton recombination radiation atan intensity ratio (FE/BE) of 0.1 or more, wherein FE represent a peakintensity of the free exciton recombination radiation, and BE representsa peak intensity of a boron-derived bound exciton recombinationradiation.